Native Proxy support

[gthieleb]

For a script within customer network there is a need for proxy support.

What works is using environment http_proxy / https_proxy which is luckely supported from the requests module. But this is kind unflexible when accessing an external and internal taiga instance at the same time.

Where is a good starting point, adding proxy support to this module?

[yakky]

@gthieleb https://github.com/nephila/python-taiga/blob/master/taiga/requestmaker.py and auth* methods (https://github.com/nephila/python-taiga/blob/master/taiga/client.py#L138 and https://github.com/nephila/python-taiga/blob/master/taiga/client.py#L182) are the points where requests is called.
Adding a proxy argument to TaigaAPI init (https://github.com/nephila/python-taiga/blob/master/taiga/client.py#L53) and the passing it to requests where needed should be the best way to achieve this

[yakky]

@gthieleb do you need any help to contribute this?